This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Silk is a biopolymer made of protein. It is synthesized by spiders and some insect larvae to fulfill various functions. According to its use, the protein composition, the structure and the mechanical properties of the fiber vary. Certain silk fibers, such as the Nephila clavipes dragline, display tensile properties comparable or even better than those of high-performance man-made fibers. However, the natural process of silk fiber formation occurs at ambient pressure and temperature and does not involve any polluting solvent, contrarily to the production of high-performance fibers. Moreover, most of these man-made fibers are toxic and extremely difficult to degrade whereas silk is bio-degradable. A detailed understanding of the natural silk formation process will enable us to mimic it so as to reach the same tailoring capabilities and environment-friendly properties. It is known from direct measurement that the pH and the ionic content of the silk protein solution changes along its transformation to a solid fiber, and that shear forces are applied. Very low pH or very high shear forces alone are able to trigger the protein amyloidal folding and aggregation characteristic of its final structure in the fiber. SAXS is used to study the effect of several cations on silk protein, such as 1 [unreadable]M to 1 mM of sodium, potassium, calcium, copper, zinc and magnesium. The concentration of all these ions is known to vary along the natural silk formation process. This study involved the application of microfluidic chips to produce fibers using buffers of different salt composition. The next step is to follow in situ the fiber formation process in presence of different cations.